This invention relates to an ionization detector which ionizes matter to be detected, detects the resulting ionization current, and indicates the concentration of the matter on the basis of the detected ionization current.
In a fast breeder reactor, liquid metal having a high seat conductivity, such as liquified sodium, is used to cool the reactor core. The liquid sodium circulates through the reactor core by means of a piping system. During the circulation via the reactor core, there may be a little leakage of sodium vapor. To secure the safety of the reactor system, such leakage must be detected as soon as possible. For the detection of the leakage of sodium, the surrounding gas in the vicinity of every branch of the piping is sampled and the sampled gas is sent to a sodium ionization detector (also referred to as SID for brevity). In the SID, the sodium particles contained in the sampled gas are ionized by the heat generated by a filament and the generated Na.sup.+ ions are collected by a collector. The number of the sodium particles can be detected by detecting the ionization current flowing between the filament and the collector. An ionization detector of this type is disclosed in the U.S. Pat. No. 4,117,396. According to that disclosure, the filament is always heated up to 800.degree.-1000.degree. C. so as to effectively ionize the sodium particles on the surface thereof. With the SID filament as disclosed in the above literature, the temperature of the filament fluctuates with the lapse of time, depending upon the changes in the flow of the gas to be sampled, the current sent through the filament to generate heat, etc. Accordingly, the ionization current also fluctuates so that it becomes difficult to exactly detect the number of the sodium particles.
Another apparatus for detecting the leakage of fluid such as, for example, liquified sodium is disclosed in the commonly assigned U.S. patent application Ser. No. 60,767 filed on July 26, 1979, now U.S. Pat. No. 4,259,861. In that apparatus, sodium leakage is detected on the basis of the phenomenon that when electrically conductive metal having a small heat capacity touches sodium in the state of mist, the temperature of the metal falls. The temperature of the electrically conductive metal is kept at a temperature through a feedback control. The change in the temperature of the metal causes the change in the electrical conductivity thereof and the latter is taken out as a pulse output. The pulse output thus obtained is subjected to a desired processing and thereafter sent to a pulse height analyzer to detect the concentration of the sodium mist. Therefore, this apparatus can make a rather exact determination of sodium leakage, but there is also need for a complicated circuit for processing the output pulse.